As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased. Among them is a lithium secondary battery having high energy density and operating voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as for the mobile devices.
Based on their external and internal structures, secondary batteries are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. Especially, the prismatic battery and the pouch-shaped battery, which can be stacked with high integration and have a small width to length ratio, have attracted considerable attention.
An electrode assembly cell having a cathode/separator/anode structure, which constitutes a secondary battery, may be generally classified as a jelly-roll (winding) type electrode assembly or a stacking type electrode assembly based on the structure of the electrode assembly. The jelly-roll type electrode assembly is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and helically winding the anode/separator/cathode structure. The jelly-roll type electrode assembly is suitable for cylindrical batteries; however, the jelly-roll type electrode assembly is not suitable for prismatic batteries or pouch-shaped batteries because the electrode active material may be detached, and the spatial utilizability is low. On the other hand, the stacking type electrode assembly is an electrode assembly constructed in a structure in which a plurality of cathode and anode unit cells are sequentially stacked one on another. The stacking type electrode assembly has an advantage in that the stacking type electrode assembly can be constructed in a prismatic structure; however, the stacking type electrode assembly has disadvantages in that a process for manufacturing the stacking type electrode assembly is complicated and troublesome, and, when external impacts are applied to the stacking type electrode assembly, electrodes of the stacking type electrode assembly are pushed with the result that short circuits occur in the stacking type electrode assembly.
In order to solve the above-described problems, there has been developed an electrode assembly having a novel structure, which is a combination of the jelly-roll type electrode assembly and the stacking type electrode assembly, i.e., a stacking-folding type electrode assembly constructed in a structure in which full cells having a cathode/separator/anode structure of a predetermined unit size or bicells having a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure of a predetermined unit size are folded using a long continuous separation film. Examples of such a stacking-folding type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application.
Generally, the number of the full cells or bicells constituting the stacking-folding type electrode assembly is increased in order to increase the capacity of the stacking-folding type electrode assembly. As the number of the full cells or bicells is increased, however, a great deal of working time is required to fold the increased full cells or bicells. In addition, when some of the cells malfunction, the electrode assembly also malfunctions.
Meanwhile, Korean Patent Application Publication No. 2004-0054201 and No. 2004-0092533 disclose technologies for connecting two or more stacking or winding electrode assemblies in parallel with each other and mounting the connected electrode assemblies into a pouch-shaped battery case or a prismatic battery case, to increase the capacity of a secondary battery, although the electrode assemblies are not constructed in the above-described stacking-folding type structure. However, these technologies have problems in that the two or more electrode assemblies are coupled to each other only at electrode terminals thereof in order to accomplish the electrical connection between the electrode assemblies, and the electrode terminals protrude from only one-side ends of the electrode assemblies, whereby the structural stability of the secondary battery is very low. This problem may act as one of the major factors to deteriorate the stability of the secondary battery when the secondary battery is exposed to external environments, such as impacts and vibrations, which are frequently generated in the secondary battery requiring high capacity.
Consequently, there is a high necessity for a technology to increase the capacity of a secondary battery and increase the electrical and physical coupling force between cells constituting the secondary battery, thereby improving the structural stability of the secondary battery.